1. Field of the Invention
The present invention relates to an EFM (Eight-to-Fourteen Modulation) signal comparator appropriate to be built into a large-scale integrated circuit.
2. Prior Art
Compact discs store data coded in the form of EFM signals. The waveform of the EFM signals read from a compact disc is shaped so that the duty cycle thereof is regulated to 50% before the signals are supplied to the main part of a CD amplifier. An EFM-signal comparator is used to convert the read data into a train of pulses having a duty cycle of 50%.
FIG. 1 shows a configuration of a conventional EFM-signal comparator. In FIG. 1, numeral 1 designates an input terminal to which input EFM signals are applied. Input terminal 1 is connected to an output terminal 5 via capacitor 2, and inverters 3 and 4. To the output terminal of inverter 3, a series of resistor 7 and capacitor 8 is connected, whereas to the output terminal of inverter 4, a series of resistor 10 and capacitor 11 is connected. The connecting point of resistor 7 and capacitor 8 is connected to the noninverting input terminal of operational amplifier 12, while the connecting point of resistor 10 and capacitor 11 is connected to the inverting input terminal of operational amplifier 12. The output terminal of operational amplifier 12 is connected to the input terminal of inverter 3 via resistor 14.
The input EFM signals transmitted from the optical pick-up are outputted from inverter 3 in the form of a square pulse train. This square pulse train has a duty cycle specified by the relationship between the EFM signal level and the input threshold level of inverter 3. The square pulse train produced from inverter 3 is smoothed by resistor 7 and capacitor 8, and an average voltage Va thereof is supplied to the noninverting input terminal of operational amplifier 12. The square pulse train is also supplied to inverter 4. The inverted signal from inverter 4 is smoothed by resistor 10 and capacitor 11, and is supplied to the inverting input terminal of operational amplifier 12.
When the duty cycle of the pulse train outputted from inverter 3 is larger than 50%, voltage Va is higher than voltage Vb, so that a positive voltage is produced from the operational amplifier 12. This positive voltage is supplied to the input terminal of inverter 3 via resistor 14. As a result, the EFM signal level at the input terminal of inverter 3 is raised, so that the duty cycle of the square pulse train at the output terminal of inverter 3 decreases, which decreases voltage Va. In short, when Va&gt;Vb, voltage Va is lowered, so that Va becomes equal to Vb. On the other hand, when the duty cycle of the pulse train produced from inverter 3 is less than 50%, voltage Va is lower than voltage Vb, so that a negative voltage is produced from operational amplifier 12. The negative voltage is supplied to the input terminal of inverter 3 via resistor 14. As a result, the EFM-signal level at the input terminal of inverter 3 is lowered, so that the duty cycle of the square pulse train produced from inverter 3 increases, which increases voltage Va. In short, when Va&lt;Vb, voltage Va is raised, so that Va becomes equal to Vb. Thus, the duty cycle of EFM signals outputted from the output terminal 5 is regulated to 50%.
The conventional EFM-signal comparator described above, however, uses a general-purpose operational amplifier 12. Consequently, the part defined by a dash-dotted line in FIG. 1 cannot be integrated into an LSI and must be placed outside the LSI. This hinders the construction of an LSI which includes the entire compact disc reproduction circuit.